Many telephone companies provide custom calling services such as Call Waiting, Repeat Dialing, Priority Ringing, Select Call Forwarding, and other services. These services involve tone signals to alert subscribers as well as manipulation of telephone line switches to ensure proper connections. Tone signals and line switches, however, may be are a serious problem for a subscriber who uses a single telephone line for both voice and data transmission.
Call Waiting is a service that alerts a single-line subscriber that another call is waiting, by interrupting a call in progress with a repeated tone burst. So the calling party won't hear the same tone burst (or echo), line switches are opened for the duration of the tone bursts. The calling party will notice just a brief interruption of the voice. Other service features use different tones and signal patterns, but they all have in common that tone signaling and line switching at least briefly interrupt a call in progress.
Since tone bursts are brief, they do not significantly effect the intelligibility of a conversation. However, considering the case of data transmission, any interruption that lasts long enough to mask bits in a digital data stream can cause major problems in data transmissions. For example, a single-line subscriber may use a line alternatively for voice calls or data-type transmission such as, but not limited to, fax or e-mail. If a Call Waiting tone burst interrupts a data transmission, data bits are lost and at least part of the data transmission is useless.
Many telephone companies provide a Suspend Call Waiting feature, which eliminates the interference problem only for an originator of a call. That is, a subscriber can temporarily suspend the Call Waiting feature by dialing a sequence of digits, such as *70 for a touch-tone phone and 1170 for a rotary phone, before dialing a telephone number. By suspending the Call Waiting feature, subscribers can send transmissions free of Call Waiting interruptions. When a call is completed, the Call Waiting feature is automatically restored.
Unfortunately, even with a Suspend feature a Call Waiting tone burst remains a serious problem in the event a data-type transmission is received, because a subscribers cannot suspend the Call Waiting feature when receiving a call.
What is clearly needed is a system that provides a Selective Call Waiting feature. This feature would function basically the same as the Call Waiting feature, but in addition, it would provide an ability to check a subscriber's telephone line to ascertain whether a call in progress is a data-type or voice-type transmission.
In Selective Call Waiting according to the present invention, if a data-type transmission is detected, the selective Call Waiting feature suspends alert signals and sends a busy signal to any calling party. If voice-type transmission is detected, the Selective Call Waiting feature alerts the called party with a brief, repeated tone and sends simultaneously a ring signal to the calling party.